TORONTO (AP) ' Serena Williams rallied for a hard-fought 4-6, 6-3, 6-2 victory over Lucie Safarova at the Rogers Cup on Friday, reaching the tournament semifinals as she continues her comeback from injury and illness.

The unseeded Williams next will play fourth-seeded Victoria Azarenka of Belarus, who made quick work of No. 135 Galina Voskoboeva of Kazakhstan with a 6-1, 6-2 victory.

The 11th-ranked Australian broke Vinci's serve while leading 5-4 in the first set to take the opener at the Rexall Centre. Stosur then poured it on against the 22nd-ranked Italian in the second to move into the semis in convincing fashion.

"As the match went on, I obviously probably grew in confidence, and then you get that bit of a lead with a buffer of a couple breaks and felt like I could maybe start going for a bit more," Stosur said. "I went for some big ones and it paid off."

"Semifinals, you've got to believe that anything in possible," Stosur said. "Having said that, from semifinals to winning the tournament, there's a still a long way to go, but I think in this position you've got to. If you don't, then it's probably a little bit silly."

Azarenka, the highest seeded player remaining in the women's draw, hasn't dropped a set in the tournament and lost just six games. She broke Voskoboeva's serve six times to win in 69 minutes. The frustration showed when Voskoboeva whacked a ball in anger, narrowly missing a ball boy.

The 5-foot-11 Azarenka said her opponent's hot streak never had her concerned.

"Worried? No. Of course, I had to take into consideration that she'd been playing really well, but I was more focused on my game and trying to execute my shots and less on the opponent's game," she said. "It really worked out well that I focused more on me than on her."

"It's one week," Azarenka said. "If she can do that on a consistent basis, she'll be up in the top 10."

The 22-year-old Radwanska took 1 hour, 41 minutes to beat Petkovic, winning the final long rally when the 23-year-old German fired a shot into the net.

Radwanska is battling a shoulder injury, which she summed up to "too much tennis." She wore a patch the size of a saucer taped over her right shoulder.

"Playing every day is not very helpful with my shoulder," Radwanska said. "Still I can handle it, and for sure I will do my best (Saturday)."

Petkovic wasn't surprised by the long rallies with Radwanska, but given the opportunity to replay the match, would have taken an entirely different approach.

"You really have to build up to point and I think I played too much flat to her, maybe I should have used the angles more, but that's something you realize after the match," Petkovic said. "Today I just felt like I needed to overpower her which was the wrong approach."

The victories for the three seeded players came just as the star power had all but fizzled out with 11 of 16 seeds gone by the end of the third round.